1. Field of the Invention
The present invention relates to a semiconductor device, in particular, to a structure of a semiconductor nonvolatile memory device and improvement of reliability thereof. The semiconductor nonvolatile memory device is mounted on the same substrate as a semiconductor device having a logic operation function, which is typified by a microcomputer.
2. Description of the Related Art
Semiconductor nonvolatile memory devices of a floating gate type are widely used. In recent years, MONOS or MNOS memory devices that store electric charge in charge trapping films are reassessed in terms of applications such as large capacity data storage and implementation on the same silicon substrate as logic semiconductor devices. Hereinafter, a single semiconductor nonvolatile memory device is referred to as a memory cell. A whole device that is comprised of a plurality of the memory cells and has a nonvolatile memory function is referred to as a semiconductor nonvolatile memory device. In relation to the present invention, for example, U.S. Pat. No. 5,408,115 showing operations for injecting and extracting electrons into/from memory cells is known. This document discloses a technique for injecting electrons into a three-layer charge storage structure consisting of an oxide film, a nitride film, and an oxide film by source-side hot-electron injection. The document also discloses a technique for extracting the electrons into a gate electrode by applying a positive voltage to the gate electrode.